1. Institute of Physiology II, University of Würzburg, Röntgenring 9, D-97070 Würzburg, Germany
2. Department of Radiation Oncology, University of Tübingen, Hoppe-Seyler-Str. 3, D-72076 Tübingen, Germany
3. Institute of Physiology, Medical Faculty, RWTH Aachen University, Pauwelsstr. 30, D-52074 Aachen, Germany